The promised Mahdi, who is usually mentioned by his title of Imam-i 'Asr (the Imam of the "Period") and Sahib al-Zaman (the Lord of the Age),is the son of the eleventh Imam. His name is the same as that of the Holy Prophet. He was born in Samarrah in 256/868 and until 260/872 when his father was martyred, lived under his father's care and tutelage. He was hidden from public view and only a few of the elite among the Shi'ah were able to meet him.
After the martyrdom of his father he became Imam and by Divine Command went into occultation (ghaybat). Thereafter he appeared only to his deputies (na'ib) and even then only in exceptional circumstances.
The occultation of the twelfth Imam is, therefore, divided into two parts: the first, the minor occultation (ghaybat-i sughra) which began in 260/872 and ended in 329/939, lasting about seventy years; the second, the major occultation which commenced in 329/939 and will continue as long as God wills it. In a hadith upon whose authenticity everyone agrees, the Holy Prophet has said, "If there were to remain in the life of the world but one day, God would prolong that day until He sends in it a man from my community and my household. His name will be the same as my name. He will fill the earth with equity and justice as it was filled with oppression and tyranny."